Production of large steel ingots with consumable vacuum arc hot tops

ABSTRACT

A method is provided for preventing shrinkage cavities and voids in steel ingots by the steps of forming a body of degassed molten steel in an ingot mold and continuously supplying molten steel by vacuum arc remelting an electrode into the body of molten steel at a controlled rate substantially equal to the volume loss by shrinkage on cooling until the body of steel is solidified.

United States Patent Cooper [54] PRODUCTION OF LARGE STEEL INGOTS WITH CONSUMABLE VACUUM ARC HOT TOPS [72] Inventor: Lloyd R. Cooper, Pittsburgh, Pa.

Assignee:

Filed:

App]. No.:

l-leppenstall Company May 18, 1971 Related U.S. Application Data Continuation-impart of Ser. No. 80,669, Oct. 14, I970, abandoned.

U.S. Cl ..164/52, 164/65 Int. Cl. .,B22d 27/02 Field of Search 164/50, 52, 61, 65, 252

[ Oct. 10, 1972 [56] 7 References Cited UNITED STATES PATENTS 2,230,296 2/ 1941 Hopkins ..l64/52 3,305,901 2/1967 Gero 164/65 3,341,321 9/1967 Morrison et al. ..75/12 Primary Examiner-J. Spencer Overholser Assistant ExaminerJohn E. Roethel Attorney-Buell, Blenko & Ziesenheim [57] ABSTRACT 7 Claims, 2 Drawing Figures PATENTEDnm 10 1912 INVENTOR y R. Cooper PRODUCTION OF LARGE STEEL INGOTS WITH CONSUMABLE VACUUM ARC HOT TOPS This application is a continuation-in-part of my copending application Ser. No. 80,669, filed Oct. 14, 1970, now abandoned.

This invention relates to methods of producing large steel ingots and particularly to a method for producing large steel ingots with consumable vacuum are hot tops to provide a large solid ingot free of central cavity or pipe which is a common problem in the casting of large ingots.

In the production of steel ingots for subsequent forging or rolling operations, the conventional method consists of teeming the liquid steel into chilled iron molds of the desired cross sectional area and height to contain the liquid steel, and to produce a solid steel ingot which can be heated and handled for the forging or rolling operation.

The volume change of steel from the liquid to the solid condition is well known as the cause of shrinkage cavities and voids in steel ingots. In fully deoxidized or killed steels, it is necessary to provide a reservoir or sinkhead of additional liquid steel on top'of the ingot body, in an effort to maintain a liquid pool for a period of time necessary to permit the liquid steel to feed into the ingot body while it issolidifying and shrinking in volume. As producers of steel ingots are painfully aware, this effort to produce sound steel ingots without shrinkage cavities is not always successful, even though the refractory insulated sinkhead on top of the cast iron mold containing the ingot body may be as much as 25 or 30 of the total teemed ingot weight. This is especially true as the ingot weight and ingot cross section are increased, and the times required for full solidification are correspondingly lengthened.

Various methods are used to keep the reservoir of steel sufficiently hot and liquid during the extended solidification period. These methods include insulating covers over the top of the sinkhead, exothermic powders and/or exothermic liners in the sinkhead to introduce additional heat to maintain liquid condition, electric induction around the sinkhead, electric are on top of the liquid steel, or electric current through liquid slag on top of the liquid steel (electro slag).

These methods are effective, to varying degrees, in feeding liquid steel into the solidifying ingot body over the period required. All of the methods, as practiced, however, have several common detrimental characteristics. First of these is that, as the liquid reservoir is maintained, it is subject to oxidation and chemical change. This is true, with or without the liquid slag cover, and becomes especially severe as progressively larger ingots require the availability of a liquid steel reservoir for as long as l hours, 20 hours or longer.

A second major detriment of the above conventional hot topping methods, is that the liquid metal for the hot top or sinkhead volume must be provided as part of the liquid steel being melted and teemed for product. This is true even though it is known that the product will be obtained only from the ingot body, and that the metal provided for the sinkhead volume will be cut from the total ingot weight and discarded as scrap steel.

The invention described here pertains to the use of a consumable steel electrode, of the same composition as the ingot body being teemed, to be melted in vacuum,

by an are passing between the bottom of the electrode and the top liquid surface of the teemed ingot. The melting rate will be controlled by the voltage and current passed through the electrode, and will be such that the shrinkage of the ingot body will be totally compensated by the liquid steel melted from the consumable electrode.

The pressure in the chamber containing the ingot body, the electrode, and the arc over the liquid pool in the ingot body will be sufficiently low to permit a stable are for melting the consumable electrode, preferably below 600 microns I-Ig pressure, according to known practice of Consumable Vacuum Arc remelting. 1n the same manner, the pressure will be sufficiently high to minimize loss of manganese, and other desired elements with high vapor pressure, from the liquid steel, preferably at least 300 microns Hg Pressure. The pressure can be controlled upward, if necessary, by introduction of an inert gas, as argon, into the vacuum system subsequent to the desired 300-600 micron Hg starting pressure range. During the course of the consumable vacuum are hot topping cycle, this pressure may be increased to several mm. Hg. pressure, and up to 10 mm. or even higher, depending on the melting rate and the arc stability.

In the foregoing general description of my invention I have set out certain problems of the prior art and certain objects, purposes and advantages of my invention in relation to those problems. Other objects, purposes and advantages of my invention will be apparent from a consideration of the following description and the accompanying drawings in which:

FIG. 1 shows a vertical section through an ingot mold and vacuum chamber with a pouring ladle for casting an ingot according to my invention.

FIG. 2 is a vertical section through the mold and chamber of FIG. 1 with a consumable electrode in place of the pouring ladle.

Referring to the drawings I have illustrated a typical production practice for my invention in which a cast iron mold 10 is placed on a cast iron stool 11 within a housing 12 forming a chamber around the mold 10. The housing 12 is provided with a port 13 connected to a vacuum pump system of usual form used in Vacuum Stream Degassing and therefore not illustrated. A cover 14 is placed on housing 12 in sealing relationship and a ladle 15 containing molten metal to be cast into mold 10 is placed on the cover. The chamber within housing 10 is evacuated to a desired low pressure and liquid steel 16 is teemed into the cast iron mold 10 according to the well known vacuum stream degassing technique. Vacuum stream degassing is a well known" practice and is used here to illustrate one of the methods of casting a large ingot of vacuum degassed liquid steel.

After teeming of the steel 16 into mold 10 is completed, atmospheric pressure is restored in the chamber within housing 12 and ladle 15 is removed. A furnace body 17 adapted to carry a vacuum tight ram 18 and holder 19 for an electrode 20 is placed on cover 14. The electrode 20 is made from the same material as steel 16. The pressure in housing 12. is again reduced to the desired starting range (e. g. 300 to 600 microns I-Ig pressure), the electrode 20 is lowered to the top of the steel 22 in the mold l0 and the arc is started, melting steel from the bottom of the electrode which falls into the body of liquid steel forming the ingot 22. Melting of the electrode 20 is continued during the entire time for complete solidification of the ingot 22 at a rate required to overcome the shrinkage in the ingot body. The level of the liquid steel pool in the top of the ingot body is monitored by appropriate sight ports 23 or by television cameras and viewing screens. The power supplied to the consumable electrode 20 is the same as normally used for consumable arc-remelting i.e. direct current, straight polarity (electrode negative) 25 to 65 volts. It is also possible that the polarity may be changed to reversed polarity (electrode positive) in order to generate additional heat in the liquid steel pool. The electrode diameter is preferably about 25 to 35 of the diameter of the ingot body and is centered over the ingot body to maintain the liquid pool in the central axial zone of the ingot body which is the last to solidify. For example, an ingot body which is 102 inches in diameter (in a round corrugated cast iron mold). and which weighs 348,000 pounds, would preferably use an electrode 25 inches to 35 inches in diameter. Such an electrode would weigh at least 17,400 pounds (5 percent of the ingot body weight teemed) in order to provide sufficient metal to overcome the shrinkage of the solidifying ingot. This 25 inch to 35 inch diameter electrode would require electric power of from 25 to 50 volts DC. and 15,000 to 25,000 amperes, in accord with known consumable vacuum arc remelting practice, with reductions in current during the final finishing cycle. At the end of the solidification period when no more steel is required in the ingot body, the operation is stopped and atmospheric pressure restored within housing 10.'The furnace body 17 is removed, the cover 14 is removed and the ingot 22 is stripped from the mold 10. The subsequent handling of ingot 22 is as required for the particular steel produced and the end sought.

- In the foregoing description a furnace body 17 is placed on top of the cover 14 and housing 12; however the furnace body 17 with an adapter plate, or a specially designed furnace body, could beequally well placed directly on top of the mold 10, and evacuated after which the electrode 20 is melted as described above. r

The outline above describes the use of the consumable vacuum are hot topping on a steel ingot that is degassed and teemed by the Vacuum Stream Degassing Methods. Other methods of degassing may also beused, as for example, by Tap Degassing, Vacuum Ladle Degassing, Ladle to Ladle Degassing, or others, including degassing the air teemed steel in the ingot mold during the pumping down cycle over the top of the mold preparatory to the consumable vacuum are hot topping operation. This latter method of degassing an otherwise undegassed steel, in the ingot mold, involves rapid and usually violent evolution of gases from the liquid steel, and requires a high "free-board areav of the ingot mold, to contain all of the actively moving liquid steel,

, during degassing.

Two major advantages are: obtained by the hot topping method described by this invention. The first, and most important advantage, is the fact that the steel ingot is permitted to solidify under the optimum conditions for center soundness and for a minimum of chemical segregation. By virtue of the vacuum are remelting of the consumable electrode, it is possible to guarantee that the reservoir of liquid steel is maintained duringthe total time period required for complete solidification of the ingot body. The maintenance of this liquid pool at the top of the ingot body itself assures that no intermediate freezing or bridging occurs in the ingot body itself and consequently secondary piping or shrinkage cavities can be eliminated.

A second important advantage of this hot topping method is that in the production of ingots of maximum sizewhich normally use all of the melting and refining capacity of a melting shop, the size range of available ingots can be extended. For example, if a particular melt shop of several furnaces can be scheduled to produce a total of 400,000 pounds of steel which may be combined to teem one ingot weighing 400,000 pounds, by normal teeming and hot topping means, the ingot body would weigh approximately 300,000 pounds. The balance would be in the hot top. A resulting forging would be a proportion of this 300,000 pound ingot body. By theinvention described here however, all of the melt shop capacity of 400,000 pounds may be teemed into a cast iron mold to make up the ingot body and the steel secured from the consumable electrode used to overcome normal shrinkage in this ingot body. In this manner all of the steel of the melt shop is used to produce the usable body of the ingot, and the resulting forging available from this ingot body would be a correspondingly greater weight. In short the productive capacity of a shop can be increased about 25 to 30 percent without changing the size of equipment used to produce the molten steel. This is a very important advantage.

In the foregoing specification l have set out certain preferred practices and embodiments of my invention, however it will be understood that this invention may be otherwise embodied within the scope of the following claims.

lclaim:

1. A method of preventing shrinkage cavities and.

voids in metal ingots comprising the steps of:

a. teeming a body of degassed molten metal into an ingot mold, and continuously supplying molten metal of like composition to said molten body before the teemed metal completely solidifies by vacuum are remelting an electrode into the body of molten metal at a controlled rate substantially equal to the volume loss by shrinkage on cooling until the body of metal is solidified in theingot mold whereby the level of molten metal in the ingot moldis maintained substantially constant until solidification is completed.

2. A method of preventing shrinkage cavities and voids in metal ingots as claimed in claim 1 wherein the body of degassed molten metal in an ingot mold is formed within a vacuum chamber by vacuum stream degassing, and molten metal is continuously supplied by vacuum arc remelting an electrode into the body of metal from a furnace mounted on said chamber.

3. A method of preventing shrinkage cavities as claimed in claim 1 wherein all of the available molten metal in a particular composition is cast in an ingot mold free of a hot top and molten metal is continuously supplied by vacuum arc remelting an electrode of substantially the same composition.

4. A method of preventing shrinkage cavities and voids as claimed in claim 1 wherein the electrode is about 25 to 35 percent of the diameter of the ingot body.

5. A method of preventing shrinkage cavities and voids as claimed in claim 1 wherein the electrode is substantially centered over the path of molten metal in the mold.

6. A method of preventing shrinkage cavities and voids in metal ingots as claimed in claim 1, and at the same time overcoming normal segregation of chemical 

1. A method of preventing shrinkage cavities and voids in metal ingots comprising the steps of: a. teeming a body of degassed molten metal into an ingot mold, and b. continuously supplying molten metal of like composition to said molten body before the teemed metal completely solidifies by vacuum arc remelting an electrode into the body of molten metal at a controlled rate substantially equal to the volume loss by shrinkage on cooling until the body of metal is solidified in the ingot mold whereby the level of molten metal in the ingot mold is maintained substantially constant until solidification is completed.
 2. A method of preventing shrinkage cavities and voids in metal ingots as claimed in claim 1 wherein the body of degassed molten metal in an ingot mold is formed within a vacuum chamber by vacuum stream degassing, and molten metal is continuously supplied by vacuum arc remelting an electrode into the body of metal from a furnace mounted on said chamber.
 3. A method of preventing shrinkage cavities as claimed in claim 1 wherein all of the available molten metal in a particular composition is cast in an ingot mold free of a hot top and molten metal is continuously supplied by vacuum arc remelting an electrode of substantially the same composition.
 4. A method of preventing shrinkage cavities and voids as claimed in claim 1 wherein the electrode is about 25 to 35 percent of the diameter of the ingot body.
 5. A method of preventing shrinkage cavities and voids as claimed in claim 1 wherein the electrode is substantially centered over the path of molten metal in the mold.
 6. A method of preventing shrinkage cavities and voids in metal ingots as claimed in claim 1, and at the same time overcoming normal segregation of chemical elements in the central zone of metal ingots wherein all of the available molten metal in a particular composition is cast in an ingot mold free of a hot top, and molten metal is continuously supplied by vacuum arc remelting an electrode of an appropriate different composition to compensate for the heterogeneous effects of selective freezing.
 7. A method of producing metal ingots according to the embodiments of claim 1 wherein the central zone of the ingot has substantially the same solidification characteristics of consumable vacuum arc remelted ingots. 